Bimetallic MnCo selenide yolk shell structures for efficient overall water splitting

Fabricating a high-performance anode by coating a carbon layer on a yolk-shell bimetallic selenide microsphere for enhanced lithium storage

The rational synthesis of an electrode material with a highly active and stable architecture is very critical to achieving high-performance electrochemical energy storage. Herein, N-doped carbon restricting yolk-shell CoSe2/Ni3Se4 (CoSe2/Ni3Se4@NC) flower-like microspheres were successfully synthesized from solid CoNi-glycerate microspheres using a coating technology as an anode material for lithium-ion batteries (LIBs). The unique yolk-shell CoSe2/Ni3Se4@NC microspheres with hierarchical pores can increase the contact area with the electrolyte and provide enough transfer channels for the diffusion of Li+. The carbon layer on the surface of CoSe2/Ni3Se4@NC can not only improve the conductivity of the electrode but also provide the protective effect of active nanosheets during the process of synthesis, avoiding the overall structure collapse during the charge/discharge process of LIBs. Benefiting from the high conductivity, hollow structure, and elastic NC shell bestowed by the unique architecture, the yolk-shell CoSe2/Ni3Se4@NC anode shows excellent lithium storage performances, such as an excellent reversible specific capacity of 319 mA h g-1 at a current density of 1000 mA g-1 after 500 cycles and excellent cycling stability. This synthesis strategy provides a new way to optimize the lithium storage performance of transition metal compound electrode materials, which is helpful to the design of the next generation of high-performance LIBs.

Scalable One-Pot Fabrication of Carbon-Nanofiber-Supported Noble-Metal-Free Nanocrystals for Synergetic-Dependent Green Hydrogen Production: Unraveling Electrolyte and Support Effects

Electrocatalytic hydrogen evolution reactions (HER) are envisaged as the most promising sustainable approach for green hydrogen production. However, the considerably high cost often associated with such reactions, particularly upon scale-up, poses a daunting challenge. Herein, a facile, effective, and environmentally benign one-pot scalable approach is developed to fabricate MnM (M═Co, Cu, Ni, and Fe) nanocrystals supported over in situ formed carbon nanofibers (MnM/C) as efficient noble-metal-free electrocatalysts for HER. The formation of carbon nanofibers entails impregnating cellulose in an aqueous solution of metal precursors, followed by annealing the mixture at 550 °C. During the impregnation process, cellulose acts as a reactor for inducing the in situ reductions of MnM salts with the assistance of ether and hydroxyl groups to drive the mass production (several grams) of ultralong (5 ± 1 μM) carbon nanofibers ornamented with MnM nanoparticles (10-14 nm in size) at an average loading of 2.87 wt %. For better electrocatalytic HER benchmarking, the fabricated catalysts were tested over different working electrodes, i.e., carbon paper, carbon foam, and glassy carbon, in the presence of different electrolytes. All the fabricated MnM/C catalysts have demonstrated an appealing synergetic-effect-dependent HER activity, with MnCo/C exhibiting the best performance over carbon foam, close to that of the state-of-the-art commercial Pt/C (10 wt % Pt), with an overpotential of 11 mV at 10 mA cm-2, a hydrogen production rate of 2448 mol g-1 h-1, and a prolonged stability of 2 weeks. The HER performance attained by MnCo/C nanofibers is among the highest reported for Pt-free electrocatalysts, thanks to the mutual alloying effect, higher synergism, large surface area, and active interfacial interactions over the nanofibers. The presented findings underline the potential of our approach for the large-scale production of cost-effective electrocatalysts for practical HER.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Research progress of yolk-shell structured nanoparticles and their application in catalysis

Yolk-shell nanoparticles (YSNs) have attracted a broad interest in the field of catalysis due to their unique structure and properties. The hollow structure of YSNs brings high porosity and specific surface areas which is conducive to the catalytic reactions. The flexible tailorability and functionality of both the cores and shells allow a rational design of the catalyst and may have synergistic effect which will improve the catalytic performance. Herein, an overview of the research progress with respect to the synthesis and catalytic applications of YSNs is provided. The major strategies for the synthesis of YSNs are presented, including hard template method, soft template method, ship-in-a-bottle method, galvanic replacement method, Kirkendall diffusion method as well as the Ostwald ripening method. Moreover, we discuss in detail the recent progress of YSNs in catalytic applications including chemical catalysis, photocatalysis and electrocatalysis. Finally, the future research and development of YSNs are prospected.

PdO@CoSe2 composites: efficient electrocatalysts for water oxidation in alkaline media

In this study, we have prepared cobalt selenide (CoSe2) due to its useful aspects from a catalysis point of view such as abundant active sites from Se edges, and significant stability in alkaline conditions. CoSe2, however, has yet to prove its functionality, so we doped palladium oxide (PdO) onto CoSe2 nanostructures using ultraviolet (UV) light, resulting in an efficient and stable water oxidation composite. The crystal arrays, morphology, and chemical composition of the surface were studied using a variety of characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy. It was also demonstrated that the composite systems were heterogeneous in their morphology, undergoing a shift in their diffraction patterns, suffering from a variety of metal oxidation states and surface defects. The water oxidation was verified by a low overpotential of 260 mV at a current density of 20 mA cm-2 with a Tafel Slope value of 57 mV dec-1. The presence of multi metal oxidation states, rich surface edges of Se and favorable charge transport played a leading role towards water oxidation with a low energy demand. Furthermore, 48 h of durability is associated with the composite system. With the use of PdO and CoSe2, new, low efficiency, simple electrocatalysts for water catalysis have been developed, enabling the development of practical energy conversion and storage systems. This is an excellent alternative approach for fostering growth in the field.

Carbon-incorporated Ni2P-Fe2P hollow nanorods as superior electrocatalysts for the oxygen evolution reaction

A rational design and cost-effective transition metal-based hollow nanostructures are important for sustainable energy materials with high efficiency. This study reports on carbon-incorporated Ni₂P-Fe₂P hollow nanorods ((Ni,Fe)₂P/C HNRs) derived from a self-template approach as efficient electrocatalysts. Initially, a Ni₂(BDC)₂(DABCO)-MOF (Ni-MOF) is converted to NiFe-PBA hollow nanorods (HNRs) through facile ion exchange which was further converted to (Ni,Fe)₂P/C HNRs via a subsequent phosphidation process. The resulting (Ni,Fe)₂P/C HNRs exhibit remarkable activity for the oxygen evolution reaction in an alkaline solution requiring a small overpotential of 258 mV to drive a current density of 10 mA cm^-2 and long-term stability with little deactivation after 40 h. (Ni,Fe)₂P/C HNRs outperform (Ni,Fe)₂P/C NPs and commercial RuO₂. The unique hollow morphology and interfacial electronic structure substantially increase the active site and charge transfer rate of our electrocatalyst, resulting in excellent OER activity and stability.

Cooperative effect of bimetallic MOF-derived CoNi(OH)2@NiCo2S4nanocomposite electrocatalysts with boosted oxygen evolution activity

Highly efficient and inexpensive electrocatalysts for oxygen evolution reaction (OER) are extensively studied for water splitting. Herein, a unique bimetallic nanocomposite CoNi(OH)₂@NiCo₂S₄nanosheet arrays derived from metal-organic-frameworks (MOFs, CoNi-ZIF) is simply fabricated on Ni foam, endowing large specific surface area and outstanding electrical conductivity. Compared with their single-metallic counterparts, the bimetallic composite displays dramatically low overpotential and small Tafel slope as well as outstanding catalytic stability. The overpoptential at 20 mA cm^-2for CoNi(OH)₂@NiCo₂S₄is only 230 mV in comparison with Ni(OH)₂@Ni₃S₂(266 mV), Co(OH)₂@Co₃S₄(294 mV) and RuO₂(η = 302 mV). First-principle calculations based on density functional theory (DFT) are carried out and reveal that the introduction of Ni in Co(OH)₂helps lowered the energy difference of ΔG_OOH*-ΔG_O*, and thereby boosting the OER reactivity. This study provides an effective approach for the rational construction of low-cost metal hybrids.

In situ encapsulation engineering boosts the electrochemical performance of highly graphitized N-doped porous carbon-based copper-cobalt selenides for bifunctional oxygen electrocatalysis

Transition-metal selenides are gaining prominence as promising electrode materials for energy storage applications owing to their low electronegativity and environment-friendliness compared with metal sulfides/oxides. Herein, a CuCoSe@NC nanocomposite with copper-cobalt selenides embedded in highly graphitized N-doped porous carbon was synthesized by an in situ encapsulation strategy with metal-organic framework crystals (CuCo-BDC) as templates followed by selenization, and used as a bifunctional electrocatalyst for Zn-air batteries in lye. The result shows that the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic activity of the optimal CuCoSe@NC-2 was enhanced, and the assembled Zn-air batteries exhibited a remarkable electrochemical performance with the use of the CuCoSe@NC-2 electrode, including a high power density (137.1 mW cm^-2) and excellent charge-discharge cycling stability, which were better than those of the Pt/C + RuO₂ electrocatalyst. Such improvement is attributed not only to the higher porosity and larger specific surface area (342 m² g^-1) of the carbon matrix, which increased the contact area with oxygen-containing species, but also the encapsulation effect of the highly graphitized N-doped carbon layer and the high content of pyridine-N species also further improved the conductivity of selenide composites. This work has introduced N-doped bimetallic selenides as an ideal candidate for bifunctional electrocatalysts.

Nickel Foam Supported NiO@Ru Heterostructure Towards High-Efficiency Overall Water Splitting

Electrocatalytic water splitting for hydrogen production from renewable energy requires the innovation of electrocatalysts with high activity and low cost. In this work, densely packed NiO@Ru nanosheets were fabricated on the surface of Ni foam through a two-step method of Ni(OH)₂ growth followed by Ru deposition. Through pair distribution function analysis from selected-area electron diffraction and X-ray photoelectron spectroscopy, the interface structure feature is revealed as a thin layer of perovskite NiRuO₃ sandwiched between NiO and Ru. The electrode exhibits high activity and durability for HER and OER, delivering a current density of 10 mA cm^-2 at a voltage of 1.55 V for overall water splitting in 1 M KOH. The excellent performance can be attributed to the intimate interface contact of NiO and Ru in addition to low charge transfer resistance and super-hydrophilic surface structure, as verified by the electrochemical impedance spectroscopy and contact-angle measurement.

CoP2/Fe-CoP2 yolk-shell nanoboxes as efficient electrocatalysts for the oxygen evolution reaction

The development of an efficient electrocatalyst is an important requirement for water splitting systems to produce clean and sustainable hydrogen fuel. Herein, we synthesized CoP2/Fe-CoP2 yolk-shell nanoboxes (YSBs) as efficient electrocatalysts for the oxygen evolution reaction (OER). Initially, zeolitic imidazolate framework-67/CoFe-Prussian blue analogue (ZIF-67/CoFe-PBA) YSBs were prepared by the reaction of ZIF-67 and [Fe(CN)6]3- ions in the presence of a small amount of water as an etching agent. The size of the CoP2 yolk depends on the amount of water. The heteronanostructure composed of the CoP2 yolk and the FexCo1-xP2 shell with a cubic shape was obtained by phosphidation of ZIF-67/CoFe-PBA YSBs. Benefiting from the unique structure and chemical composition, the CoP2/Fe-CoP2 YSB electrocatalyst has a large specific surface area of 114 m2 g-1 and shows superior electrocatalytic performances for the OER such as a low overpotential of 266 mV, a small Tafel slope value of 68.1 mV dec-1, and excellent cyclic stability.

A Mini Review on Yolk-Shell Structured Nanocatalysts

Yolk-shell structured nanomaterials, possessing a hollow shell and interior core, are emerging as unique nanomaterials with applications ranging from material science, biology, and chemistry. In particular, the scaffold yolk-shell structure shows great promise as a nanocatalyst. Specifically, the hollow shell offers a confined space, which keeps the active yolk from aggregation and deactivation. The inner void ensures the pathway for mass transfer. Over the last few decades, many strategies have been developed to endow yolk-shell based nanomaterials with superior catalytic performance. This minireview describes synthetic methods for the preparation of various yolk-shell nanomaterials. It discusses strategies to improve the performance of yolk-shell catalysts with examples for engineering the shell, yolk, void, and related synergistic effects. Finally, it considers the challenges and prospects for yolk-shell nanocatalysts.

Fabrication of Nonmetal-Modulated Dual Metal-Organic Platform for Overall Water Splitting and Rechargeable Zinc-Air Batteries

The fast development of portable water-splitting devices has led to a great deal of work on rechargeable metal-air batteries or solar cells; however, the lack of affordable multifunctional electrocatalysts still hampers their widespread applications. Herein, a well-aligned ternary metal (oxy)hydroxide nanostructure is a sacrificial pseudomorphic transformation template of an integrated metal-organic network on the carbon cloth (CC) surface, that is, the Fe-doped metal-organic framework (MOF) ZnNiCoSe@CC nanosheet network, exhibiting powerful and efficient multifunctional electrocatalysts such as the oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction in alkaline media combined with desirable electrode kinetics. As a proof-of-concept observational study, the nanostructured Fe-doped MOF ZnNiCoSe@CC could be used as air-cathode materials in the rechargeable metal-air battery. The fabricated device delivered higher open-circuit voltage, higher capacity, and peak power density, excellent discharge-charge performance, and long cycle life. Thus, our research creates a unique perspective on the development of highly portable air electrodes with a favorable electrocatalytic application of overall water-splitting reaction.

Cobalt-based heterogeneous catalysts in an electrolyzer system for sustainable energy storage

Nowadays, the production of hydrogen and oxygen focuses on renewable energy techniques and sustainable energy storage. A substantial challenge is to extend low-cost electrocatalysts consisting of earth-abundant resources, prepared by straightforward approaches that display high intrinsic activity compared to noble metals. The expansion of bifunctional catalysts in alkaline electrolytes for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) has become very crucial in recent times. Herein, the recent progress in cobalt-based HER-OER electrocatalysts has been are brushed up and numerous bifunctional cobalt-based catalysts such as cobalt-oxides, phosphides, sulfides, selenides, nitrides, borides, carbides, perovskites, and MOF-based cobalt analogs have been investigated in detail. Specifically, much more attention has been paid to their structural variation, bifunctional activity, overpotential of the overall system, and stability. Cobalt-based catalysts with lower cell voltage, remarkable durability, and unique electronic structures, offer a new perspective in energy-related fields. In recent years, cobalt-based analogs with diagnostic facilities have been introduced due to their electronic structures, tunable d band structures, and tailorable active sites. This perspective also elucidates the present issues, promising ideas, and future forecasts for cobalt-based catalysts. The critical aspects of cobalt-based catalysts and the numerous opportunities, as discussed at the end, can possibly enrich the sustainable energy field.

Formation of uniform porous yolk-shell MnCo2O4 microrugby balls with enhanced electrochemical performance for lithium storage and the oxygen evolution reaction

Mixed transition metal oxides with favorable electrochemical properties are promising electrode materials in energy storage and conversion systems. In this work, uniform porous yolk-shell MnCo₂O₄ (denoted as YSM-MCO) microrugby balls have been synthesized by simple annealing treatment of metal carbonates with a microrugby ball shape in air. Benefiting from the desired porous structure and composition, the as-synthesized YSM-MCO exhibits enhanced electrochemical performance when investigated as anode materials for lithium-ion batteries and electrocatalysts for the oxygen evolution reaction. The YSM-MCO demonstrates remarkable lithium storage properties with a good cycling stability (94% capacity retention over 200 cycles at 0.5 A g^-1) and superior rate capability (414 mA h g^-1 at 5 A g^-1). In addition, the YSM-MCO also exhibits better OER activity than most of the reported MnCo₂O₄-based electrocatalysts.

Prussian blue analogues-derived bimetallic iron-cobalt selenides for efficient overall water splitting

The efficient and durable catalysts toward electrochemical water-splitting plays significant role in clean and renewable energy storage applications. Herein, we design the Prussian blue analogues precursor by self-assembled strategy and converted it to (Fe-Co)Se₂ composite by post-selenization method. Benefiting of unique porous morphology, high electrochemically active surface area and fast electron transfer ability, the excellent oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) performance of (Fe-Co)Se₂ catalyst were obtained. It was revealed that only require overpotentials of 251 mV to arrive a current density of 10 mA cm^-2 in OER system, and 90 mV was accomplished at a current density of 10 mA cm^-2 toward the HER activity, along with low Tafel slope of 47.6 and 58.7 mV dec^-1 was achieved for OER and HER, respectively. The (Fe-Co)Se₂ catalysts with high electrochemical activity at the same time long-time durability may encourage more practical catalytic applications for renewable energy technologies.

Mn-doped CoSe2 nanosheets as high-efficiency catalysts for the oxygen evolution reaction

This work establishes Mn–CoSe₂ nanosheets as promising OER electrocatalysts, but also opens an exciting new avenue to enhance their electrocatalytic activity by heteroatom doping.